In general, a sintered ore serving as a main raw material of a blast furnace iron-making method is manufactured through steps illustrated in FIG. 1. The raw material of the sintered ore includes iron ore powder or sintered ore undersize powder, recovery powder occurring in an ironwork, CaO-based auxiliary raw material containing limestone, dolomite or the like, granulating aids such as quick lime, coke breeze, anthracite or the like, and these raw materials are cut on a conveyor at a predetermined rate from each of a hopper 1, . . . . An appropriate amount of water is added to the cut raw materials by drum mixers 2 and 3 or the like, and the cut raw materials are mixed and granulated and converted into sintering raw material as quasi-particles having an average diameter of 3 to 6 mm. Thereafter, the sintering raw materials are charged onto an endless moving type sintering machine pallet 8 from surge hoppers 4 and 5 disposed on the sintering machine via a drum feeder 6 and a cutting chute 7 at a thickness of 400 to 800 mm to form a charged layer 9 that is also referred to as a sintering bed. Thereafter, by igniting the carbon material of the charged layer surface in an ignition furnace 10 provided above the charged layer 9, and by sucking air above the charged layer downward via a wind box 11 disposed directly below the pallet 8, the carbon material in the charged layer is combusted, and the sintering raw material is melted by combustion heat generated at this time to obtain a sintered cake. Thereafter, the sintered cake thus obtained is crushed and regulated in particle size, and agglomerate of about 5 mm or more is recovered as a finished product sintered ore and supplied to the blast furnace.
In the above-described producing process, the carbon material in the charged layer ignited by the ignition furnace 10 then continues to combust by the air sucked toward a lower layer from an upper layer within the charged layer, thereby forming a combustion and melting zone (hereinafter, also simply referred to as “combustion zone”) having a width in a thickness direction. Since a molten portion of the combustion zone inhibits the flow of the sucked air, it becomes a factor that the sintering time is extended and productivity decreases. Also, over time, that is, along with the movement of the pallet 8 to the downstream side, the combustion zone is gradually shifted to the lower layer from the upper layer of the charged layer, and after the combustion zone has passed, a sintered cake layer (sintered layer) in which the sintering reaction is completed is generated. Also, as the combustion zone is shifted to the lower layer from the upper layer, water contained in the sintering raw material is vaporized by combustion heat of a carbon material, and concentrated in the sintering raw material of the lower layer in which the temperature has not risen yet, thereby forming a wet zone. When the water concentration reaches a certain level or higher, a void between the sintering raw material particles serving as a flow passage of the suction gas is filled with water, and similarly to the melt zone, this becomes a factor that increases the air-flow resistance.
FIG. 2 illustrates distributions of a pressure loss and a temperature within the charged layer when the combustion zone moving in the charged layer having the thickness of 600 mm is located at a position (below 200 mm from the charged layer surface) of about 400 mm on the palette in the charged layer, and illustrates that the pressure loss distribution at this time is approximately 60% in the wet zone and is approximately 40% in the combustion zone.
Meanwhile, in general, an amount of production (t/hr) of the sintering machine is determined by productivity (t/hr·m2)×sintering machine area (m2). In other words, the amount of production of the sintering machine changes by a machine width or a machine length of the sintering machine, a thickness of the raw material charged layer, a bulk density of the sintering raw material, a sintering (combustion) time, an yield or the like. Therefore, in order to increase the amount of production of sintered ore, it is believed that it is effective to reduce the sintering time by improving air permeability (pressure loss) of the charged layer, or alternatively, to improve the yield by increasing the cold strength of the sintered cake before crushing.
FIG. 3 illustrates the transitions of the temperature and the time at a point in the charged layer when productivity of the sintered ore is high and low, that is, when the pallet movement speed of the sintering machine is high and low. The time kept at a temperature of 1200° C. or higher, at which the sintering raw material starts to melt, is represented by T1 in the case of low productivity and represented by T2 in the case of high productivity. Since the movement speed of the pallet is high when the productivity is high, the high-temperature zone retention time T2 becomes shorter as compared with the time T1 when the productivity is low. The retention time at a high temperature of 1200° C. becomes shorter, the combustion is insufficient, the cold strength of sintered ore is lowered, and the yield is lowered. Therefore, in order to manufacture the high-strength sintered ore with a high yield and with good productivity in a short time, by taking some means, it is necessary to extend the time kept at a high temperature of 1200° C. or higher and increase the cold strength of the sintered ore.
FIG. 4 is a diagram schematically illustrating a process in which the carbon material of the charged layer surface ignited in the ignition furnace continues to combust by the sucked air to form a combustion zone, the combustion zone sequentially moves from the upper layer to the lower layer of the charged layer, and a sintered cake is gradually formed. In addition, FIG. 5(a) is a diagram schematically illustrating each of the temperature distributions when the combustion zone is present within each layer of an upper layer part, an intermediate layer part, and a lower layer part of the charged layer illustrated within a bold frame in FIG. 4. The strength of the sintered ore is affected by the product of the temperature to be maintained at a temperature of 1200° C. or higher and the time. The greater the value is, the higher the strength of the sintered ore is. Since the combustion heat of the carbon material of the upper layer part of the charged layer is preheated by being carried by the sucked air, the intermediate layer part and the lower layer part of the charged layer are kept at a high temperature for a long time, and meanwhile, in the upper layer part of the charged layer, the combustion heat is insufficient as much as a level that is not preheated, and the combustion melting reaction required for sintering (sintering reaction) is liable to be insufficient. As a result, as illustrated in FIG. 5(b), in the yield distribution of the sintered ore in the width direction cross-section of the charged layer, the yield is lowered in the upper layer part of the charged layer. Also, both width end portions of the palette cannot ensure a sufficient retention time at a high-temperature zone required for sintering by radiation from the palette side wall and the excessive cooling due to the large amount of passing air, and after all, the yield is lowered.
To solve these problems, conventionally, a method of increasing the carbon material (coke breeze) added to the sintering raw material has been performed. However, by increasing the amount of addition of coke, as illustrated in FIG. 6, it is possible to increase the temperature in the sintered layer and to extend the time kept at 1200° C. or higher. However, at the same time, the highest achieving temperature while sintering exceeds 1400° C., which leads to decreases in reducibility of sintered ore and the cold strength by reasons to be described below.
Non-Patent Document 1 discloses tensile strength (cold strength) of various minerals generated in the sintered ore during the sintering process and the reducibility as in Table 1. Moreover, FIG. 7 illustrates that, during the sintering process, melt starts to be generated at 1200° C., and at the highest strength in the constituent mineral of the sintered ore, calcium ferrite having relatively high reducibility is generated. This is the reason that 1200° C. or higher is required as a sintering temperature. However, when the temperature rise further proceeds in excess of 1400° C., and precisely exceeds 1380° C., calcium ferrite starts to be decomposed into amorphous silicate (calcium silicate) having the lowest cold strength and reducibility, and skeleton crystal-like secondary hematite liable to be reduced and powdered. Also, from the results of the mineral synthetic test, as illustrated in the state diagram of FIG. 8, since the secondary hematite serving as an origin of reduction powdering of sintered ore rises to Mag.ss+Liq.zone, and is precipitated upon cooling, it is important to produce the sintered ore via a path of (2) rather than a path of (1) illustrated in the state diagram in suppressing the reduction powdering.
TABLE 1Type of mineralTensile strength (MPa)Reducibility (%)Hematite4950Magnetite5822Calcium ferrite10235Calcium silicate193
That is, Non-Patent Document 1 discloses that, in securing the quality of sintered ore, very important management items are controls of the highest achieving temperature during combustion and the high-temperature zone retention time, the quality of the sintered ore is substantially determined by the controls. Therefore, in order to obtain the sintered ore with high strength, excellent reducibility, and excellent reduction powdering characteristics (RDI), it is important not to decompose calcium ferrite produced at a temperature of 1200° C. or higher into calcium silicate and secondary hematite, and in order not to do that, it is necessary to keep the temperature of the charged layer to 1200° C. (solidus temperature of calcium ferrite) or higher in a state in which the highest achieving temperature in the charged layer while sintering does not exceed 1400° C., preferably, does not exceed 1380° C. Hereinafter, in the present invention, the time kept at the temperature zone of 1200° C. or higher and 1400° C. or lower is referred to as a “high-temperature zone retention time”.
In addition, conventionally, some techniques for keeping the upper layer part of the charged layer at a high temperature for a long time have been suggested. For example, Patent Document 1 suggests a technique that injects the gaseous fuel onto the charged layer after ignition to the charged layer, Patent Document 2 suggests a technique that adds a flammable gas into air sucked into the charged layer after ignition to the charged layer, Patent Document 3 suggests a technique that disposes a hood over the charged layer for increasing the temperature in the charged layer of the sintering raw material, and blows a mixed gas of air and the coke furnace gas from the hood at a position immediately after the ignition furnace, and Patent Document 4 suggests a technique that simultaneously blows the low-melting point solvent, the carbon material, and the combustible gas at the position immediately after the ignition furnace.
However, since these techniques use the gaseous fuel of high concentration and do not reduce the amount of carbon material when blowing the gaseous fuel, the highest achieving temperature in the charged layer while sintering becomes a high temperature exceeding 1400° C., as an upper limit temperature for the operating control, calcium ferrite generated in the sintering process is decomposed, the sintered ore having low reducibility and cold strength is generated and the gaseous fuel supply effect cannot be obtained, the air permeability is degraded by the temperature rise due to combustion of gaseous fuel and the thermal expansion, productivity is lowered, and further, there is a risk of causing a fire in an upper space of the sintering bed (charged layer) by the supply of gaseous fuel. Accordingly, none of the techniques leads to the practical use.
Accordingly, as a technique for solving the above-described problems, the inventors have developed a technique in which, after reducing the amount of carbon material to be added to the sintering raw material, the downstream of the ignition furnace of the sintering machine and the upper layer part of the charged layer lacking in the amount of heat required for sintering cause a sintering reaction, in a first half of the machine length of the sintering machine, various gaseous fuel diluted to the lower limit concentration of combustion or lower is introduced into the charged layer from the palette top, and the fuel is combusted inside the charged layer, thereby controlling both the highest achieving temperature in the charged layer and the high-temperature zone retention time within an appropriate range, and suggest the technique to Patent Documents 5 to 7.
In applying the techniques of the above-described Patent Documents 5 to 7 to the method for producing the sintered ore to reduce the amount of addition of carbon material to the sintering raw material, when the gaseous fuel diluted to a lower limit concentration of combustion or lower is charged into the charged layer and the gaseous fuel is combusted inside the charged layer, as illustrated in FIG. 9, since the gaseous fuel is combusted inside the charged layer (inside the sintered layer) after the carbon material is combusted, it is possible to expand the width of the combustion and melting zone in the thickness direction in a state in which the highest achieving temperature of the combustion and melting zone does not exceed 1400° C., thereby being able to effectively extend the high-temperature zone retention time.